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Zebra Mussels (Dreissena polymorpha)

What are zebra mussels?

Zebra mussels are freshwater bivalves (relatives of clams)
and are appropriately named for the alternating wavy dark
(black/brown/green) and light (yellow, cream, white) bands on
their shells, which vary or can be absent. Adults are D-shaped
and generally thumbnail-sized but can reach a maximum size of
about two inches. While these bivalves are relatively
short-lived (typically 2-3 years), they are extremely prolific
breeders, which contributes to their ability to spread rapidly
and abundantly. An adult female zebra mussel can produce about
30,000-40,000 eggs each reproductive cycle and upwards of 1
million each year. Free-swimming microscopic larvae, called
veligers, will drift in the water for several weeks and then
settle onto any hard surface they can find. Zebra mussels also
can tolerate a wide range of environmental conditions and adults
can even survive out of water for about 7 days. Unlike other
freshwater bivalves, zebra mussels are able to attach to
surfaces using their byssal threads (see photo) and form large
clusters similar to oyster beds. It is not uncommon to find
populations with densities of over 100,000 individuals per m2,
with some reaching up to 1 million per m2. (See
the lifecycle of a zebra mussel).

Where can they be found?

Zebra mussels are native to the Caspian, Black and Azov seas
of eastern Europe. This exotic species was first discovered in
the U.S. in Lake Saint Clair, Michigan in 1988 and is believed
to have been introduced in 1986 through ballast water discharge
from ocean-going ships. Since their initial discovery, zebra
mussels have spread rapidly throughout the Great Lakes and
Mississippi River Basin states and other watersheds throughout
the eastern and central U.S. (View
zebra mussel distribution progression [nationalatlas.gov]).
In August of 2002, zebra mussels were documented for the first
time in Virginia. This infestation occurred in an abandoned
quarry in Prince William County, which is used for instructional
and recreational scuba diving. (View
underwater video from quarry - Windows Media 950 KB). The
primary method of overland dispersal by zebra mussels is through
human-related activities. Given their ability to attach to hard
surfaces and survive out of water, many infestations have
occurred by adults hitching rides on watercraft. The microscopic
larvae also can be transported in bilges, ballast water, live
wells, or any other equipment that holds water. With discovery
of this population in Millbrook Quarry, transport on dive gear
has emerged as a major concern as well.

Why be concerned?

Zebra mussel invasions have caused tremendous economic and
ecological damage. Many water treatment and power facilities
must now treat their systems to keep them free of zebra mussels,
beaches must be periodically cleaned of decaying masses of dead
zebra mussels, and bottom-dwelling organisms and fisheries have
been negatively impacted. In the United States, Congressional
researchers estimated that zebra mussels cost the power industry
alone $3.1 billion in the 1993-1999 period, with their impact on
industries, businesses, and communities over $5 billion. In
Virginia, the Millbrook Quarry population posed a substantial
and immediate threat. While isolated, the quarry is separated
from Broad Run by a mere 200-300 feet and is actively used as a
dive site. Lake Manassas, just 5-1/2 miles downstream of the
quarry, serves as the primary water supply for the City of
Manassas and a number of municipalities in the area. Just
downstream of Lake Manassas is the Occoquan Reservoir, the
primary water supply for over 1 million people in northern
Virginia. If zebra mussels escaped to the adjoining waters, it
is estimated that treatment at these water facilities would cost
approximately $500,000 - $850,000 per year for chemicals and
system maintenance.

Zebra mussels also represent a significant
threat to the Commonwealth's native ecology and wildlife
communities. Zebra mussels are extremely efficient filter
feeders. One adult can filter about a quart of water per day.
While water clarity is improved as they filter out microscopic
organisms, zebra mussels effectively remove the food base upon
which many of our native fish and freshwater invertebrates are
dependent. Because of their ability to rapidly colonize any hard
surface, zebra mussels also pose a significant risk to many
bottom-dwelling species, including our rare and endangered
freshwater mussel populations. Zebra mussels can attach
themselves to the native mussel shells, making it impossible for
these animals to open or close, burrow in the stream bottom, or
feed. Zebra mussels also can damage boat hulls and engines.

What can you do?

Because of their potential impacts, VDGIF worked with a
number of state, federal, and local agencies to eradicate the
zebra mussel population at Millbrook
Quarry, and confirmed the successful eradication of the
infestation in May 2006. Unfortunately, the likelihood of future
introductions is significant, and zebra mussels have few natural
predators which are native to Virginia, or plentiful enough to
have a significant impact on zebra mussel infestations.
Therefore, it is up to each of us to take extra precautions to
stop the spread of zebra mussels at Virginia's borders! The
following actions should be taken with any equipment used in
potentially infested waters:

All equipment (e.g., dive gear, boats, trailers, motors,
etc.) should be visually and tactically inspected for the
presence of zebra mussels prior to and after use in any
water body. Additionally, any vegetation attached to this
equipment must be removed and left at the site of origin.

Remove all sediment and gritty organic materials; these
could actually be zebra mussel veligers (juveniles).

Clean and scrub boat hulls, motors, anchors and
trailers, then hose equipment with hot (140° F) and/or
high-pressure water, keeping in mind appropriate water
conservation requirements. Bilges, live wells, and any other
compartments that could hold water should be drained at the
site of origin, and, if possible, flushed with disinfectant
or hot water. All boat equipment should be allowed to remain
completely dry for at least 24 hours before being used
again.

Thoroughly clean all diving equipment in a saltwater
bath (1/2 cup per gallon) or with warm tap water (104
degrees Fahrenheit). Ensure that all equipment remains
completely dry for at least 24 hours before being used
again. Pay special attention to those areas and equipment
that can hold water.

Take similar precautions with waders, bait buckets, and
other equipment that can hold water or comes into contact
with water.

If you believe you have seen or found a zebra mussel,
please contact VDGIF immediately at 804-367-6913.

For additional information regarding zebra mussels, check the
following site:

Report Zebra Mussel Sightings!

The Virginia Department of Game and Inland Fisheries needs
your help in the continued fight against the invasive exotic
zebra mussel! Though Virginia completed the
first successful open waterbody
eradication of a zebra mussel infestation in early 2006,
monitoring the state's many waterways is necessary to document
our zebra mussel-free status and for early detection of new
populations. If you boat, fish, swim, dive, or spend time on
Virginia's lakes, rivers, stream, ponds, or quarries, please
complete the zebra
mussel survey form (PDF) and return it to us. Your
assistance is greatly appreciated!